The present invention relates generally to refractory compositions and articles made therefrom useful in metallurgical applications. More particularly, the invention relates to improved refractory compositions and bodies comprised of said compositions which can withstand the rigorous environment of molten metal and slag such as encountered during the continuous casting of molten steel. In the continuous casting of steel, special high refractory ceramic parts are employed to control the flow of molten metal and to protect the melt from oxidation as it is poured from ladles to tundishes and thence to the continuous casting molds. These refractory components include slidegate plates and stopper rods used to control the flow of molten metal, various collector nozzles in ladles and tundishes, and protective ladle shrouds or submerged pouring nozzles employed to protect the molten steel from oxidation during the metal transfer and casting operations. Naturally, these refractory articles are subjected to very harsh operating conditions, such as severe thermal shock during start-up and later exposure to the erosive and chemical attack of molten steel and slag. All of the refractory elements are subjected to the erosive effects of the molten media with the most severe attack occurring on the outer surface of the submerged pouring shroud or nozzle. This area is in direct contact with the chemically active molten slag layer which floats on the surface of the molten steel in the continuous casting mold. The area of the nozzle or shroud which contacts the slag/metal interface is known as the slagline and has long been an area of concern for both steel makers and refractory manufacturers because of the excessive chemical attack which occurs in this area.
Specialty refractories have been developed for use in steel casting operations containing elemental carbon from such sources as graphite, pitch or coke as a carbonaceous binder or as a further addition. It is known that the inclusion of elemental carbon is beneficial in protecting the finished refractory from chemical attack and erosion by molten metals and accompanying slags. Unfortunately, the carbon constituent itself is susceptible to oxidation and erosion by molten steel and common slags employed therewith. It is common practice to use a small amount of an antioxidant material, commonly referred to as glass formers, to protect the carbon from oxidation. Such known antioxidants include silicon carbide, silica, borax or other boron containing compounds in an amount of about 2-8% by weight. Common carbon bonded ceramic materials which are used in continuous steel casting applications include refractory grains and/or powders of aluminum oxide, zirconium oxide, clays, magnesium oxide, silicon carbide, silica and other dense grain such as SiAlON.
It is also common practice to manufacture refractory bodies, such as submerged pouring shrouds or nozzles as a composite structure in order to increase the service life of the article. For example, the body of the shroud may be of a carbon bonded alumina and graphite refractory material having an erosion resistant slagline sleeve section formed of a carbon bonded zirconia and graphite refractory material. The body and sleeve are copressed and then fired. The sleeve is located in the region where the submerged nozzle is in contact with the slag/metal interface. The copressed zirconia graphite slagline sleeve exhibits improved slag erosion resistance compared with nozzles with are entirely of carbon bonded alumina graphite refractory.
It has also been observed that the erosion resistance is increased by decreasing the porosity of the ceramic body, i.e., by making the fired body more dense in order to improve its resistance to high temperature gaseous and liquid phase attack. Unfortunately, when the porosity of a ceramic body is increased to improve erosion resistance, there is generally a concurrent decrease in the thermal shock resistance of the body. This has been a problem in currently known materials and continues to perplex those who wish to formulate a material system which is capable of withstanding the corrosive and erosive effects of molten steel and slag and to resist oxidation while also possessing the desired thermal shock resistance properties.
Prior attempts to improve the erosion and oxidation resistance in carbon containing refractories are exemplified in commonly assigned U.S. Pat. No. 4,540,675 to Morris, et al. which proposes the use of boron carbide as an addition in an elemental carbon containing refractory composition. Commonly assigned U.S. Pat. No. 4,877,705 to Polidor teaches a submerged pouring nozzle having a plasma sprayed slagline sleeve of a fused refractory oxide material such as stabilized zirconia, zircon, alumina and like refractory materials. U.S. Pat. No. 4,871,698 to Fishler, et al. and also commonly assigned to the assignee of the present application, discloses slagline sleeve materials comprising pressed and fired mixtures of SiAlON and/or silicon nitride and carbon which exhibit superior slag resistance. In addition, U.S. Pat. No. 4,210,264 teaches the use of silicon carbide in carbon based refractories; German Patent No. 2,724,430 teaches that silicon carbide, zirconium di-boride, and boron nitride in finely divided form are suitable as partial or complete replacements of the graphite or amorphous carbon in carbon based refractories. Hence, it appears that the use of antioxidants, including zirconium di-boride, is known in the art to protect carbon containing refractories from oxidation.
A still further problem encountered in the continuous casting of steel, and particularly so-called aluminum killed steel, is the phenomenon of alumina build up in the interior bore of the pouring nozzle or shroud. This common occurrence proceeds with time and if unchecked will cause the pouring nozzle to become completely clogged which results in a premature stoppage of the casting campaign. Special bore liners of various refractory compositions have been proposed to prevent and/or minimize the formation of alumina in the pouring nozzle or shroud bore during the casting of aluminum killed steels. Exemplary of such liner compositions are those disclosed in commonly assigned U.S. Pat. No. 4,870,037 to Hoggard, et al., which teaches the advantages of a liner made from a carbon bonded SiAlON-graphite material. Commonly assigned U.S. Pat. No. 4,913,408 to Hoggard, et al. teaches liner compositions consisting essentially of carbon and a composite selected from the group consisting of zirconia and O'SiAlON and zirconia and silicon oxynitride as effective in preventing alumina formation during the casting of aluminum killed steels.
In aluminum killed steels, the aluminum addition reacts with the dissolved oxygen or iron oxide to form finely dispersed aluminum oxide, some of which floats into the slag above the molten steel and some of which remains as highly dispersed micro-particles in the solidified steel. During continuous casting, this extremely fine alumina has a tendency to precipitate out of the molten steel on to the cooler refractory surfaces or to react or stick to the ceramic refractories that line the molten steel path from the ladle to tundish to the casting mold. The precipitated alumina has a particular affinity to the typical carbon-bonded alumina-graphite refractories utilized as ladle shrouds and submerged pouring nozzles. The alumina will continue to build up in the pouring nozzle until the flow of molten steel is reduced to a point that the nozzle must be lanced by an oxygen torch, or discarded. If oxygen lancing becomes necessary, the casting process is disrupted, costing time and money. When this occurs, casting efficiencies decrease, which causes a subsequent downgrading of the quality of the steel. A total alumina blockage of a sub-entry nozzle also decreases the expected life of the refractories and is very costly to steel producers.
The particular refractory compositions of the present invention can be formed as interior liners on submerged pouring nozzles, ladle shrouds, collector nozzles and the like to provide a surface for resisting the build up of alumina and other oxides during the casting of aluminum killed steel. In addition, the compositions of the present invention are useful as various casting components such as slagline sleeves which offer superior resistance to erosion, thermal shock and oxidation resistance so as to increase the useful life of the refractory components, for example, a pouring nozzle .